Yeah there’s a hole in my soul

For inexplicable reasons, every person in the world is born with a huge hole right in the center of their chest.
Not particularly uncomfortable, but still quite unsightly, people are constantly trying to fill it with something.
Someone fills it with religion, someone buys a bunch of stuff, and someone even stuffs other people into it.
I left mine empty because I noticed one interesting thing: if you run into the wind without bending, a cool whistling sound is heard from the hole.